Fossils Show Earlier Origin of Life on Earth
Scientists have discovered fossils of multicelled organisms that are nearly 3.5 billion years old, a finding that pushes the origin of life back several hundred million years--to the time when the Earth was but an infant.
The organisms had already evolved substantially when the Earth was no more than about 1 billion years old, so more primitive forms of life must have originated hundreds of millions of years earlier than previously thought, according to J. William Schopf, director of the Center for the Study of Evolution and the Origin of Life at UCLA.
Profound Implications
The discovery has profound implications for the theories of the origin of life, suggesting that “it must be a pretty easy thing†for life to originate and evolve even under the most primitive conditions, Schopf said in an interview.
The microorganisms, consisting of an ancient equivalent of blue-green slime that formed in muddy ponds in western Australia, “look pretty complex,†he added.
“It’s really quite surprising, because it looks as though things had progressed much earlier than we had thought,†he said. “I think what it means is that the origin of life must have occurred substantially earlier, maybe 4 billion years ago.â€
The Earth is believed to be about 4.5 billion years old, and before Schopf began his pioneering work two decades ago, scientists had thought that life began here only about 1 billion years ago.
“It looks as though it happened relatively fast, in a few hundred million years,†Schopf said. That would suggest that if there are other solar systems with planets similar to the Earth, it is reasonable to expect that life has evolved there as well. “If it’s easy here, it’s easy there,†he said.
The fossils also argue against the theory that life first arrived on this planet from somewhere else in the universe, possibly embedded in meteors, Schopf said. The record now shows that primitive life had already begun to evolve when the Earth was very young, suggesting that it originated here rather than on some distant celestial body, he said.
“Now if we found a dinosaur sitting in those rocks, it would really be interesting,†he said, because it would show that more complex life forms were on the scene before the most primitive organisms began to form in the bogs of western Australia.
Oxygen Producers
What especially intrigues Schopf about his latest finding is that the ancient fossils appear to have been oxygen producers, possibly marking the birth of organisms that produced the oxygen in the Earth’s atmosphere that made it possible for this planet to support life as it is known today.
Schopf and one of his former graduate students, Bonnie M. Packer, presented their findings in today’s issue of Science, the weekly journal of the American Assn. for the Advancement of Science. They say the evidence supports their conclusion that the fossils probably produced oxygen, but they admit that it is inconclusive.
“The best guess is that these were oxygen-producing bacteria,†he said. “You have lots of lines of evidence, and all of them are consistent with oxygen production.â€
In the end, he said, the best evidence is that the fossils appear to be very similar to organisms that still exist and are known to produce oxygen. And the evidence also shows they were equipped to do the job, he said.
“These guys (the fossils) developed the biochemistry to give off oxygen†at a time when the atmosphere did not have oxygen, Schopf said.
Early Life Complex
The finding is important because it shows that early life was far more complex than had been thought, he said.
Another scientist familiar with Schopf’s work said the fossils are “remarkably well preserved.†David Des Marais, a geochemist with Ames Research Center in Mountain View, Calif., who is studying the chemical nature of early life forms, said Schopf’s findings “appear to be a solid contribution.â€
He said the findings support the contention that life “was more diverse and complicated than had been our impression five years ago,†but he argued that it is impossible to determine how long it took to evolve into the filamented structures Schopf has discovered.
“At 3.5 billion years you drop off the edge of the table,†and the fossil record appears to die out, Des Marais said. That is because older rocks were heated by such things as volcanic activity to the point that any record was destroyed, he added.
14-Month Study
A major program aimed at filling in some of those gaps is starting this weekend under Schopf’s direction. The 14-month study will involve 50 scientists from 10 nations, representing a wide range of disciplines. The study, which builds on work that Schopf and several colleagues have been conducting over the last decade, is being financed by the National Science Foundation, the National Aeronautics and Space Administration and the National Geographic Society.
When Schopf began his work in the early 1960s, he had thought that he would be able to “go back through time†and find the point where fossils first emerged, which he thought had occurred about 1 billion years ago. But during the last two decades, fossil discoveries have pushed the record back to the Earth’s early history.
Schopf and several colleagues have concentrated their search on two areas in South Africa and Australia known to have some of the oldest rocks ever found. Earlier discoveries in both locations yielded promising, but inconclusive, results. In 1982, they began a systematic search of an ancient hill in western Australia that geochronologists had dated at 3.5 billion years. They collected various samples and returned to the UCLA laboratories.
When they put their samples under the microscope, they found a treasure trove, Schopf said.
‘You Can See Growth’
“These are not simple cells,†he said. “They are differentiated into filaments and colonies. These are (fossils of) nice, living organisms. You can see growth and the sort of complexity we see today. So that says there’s life there. Not only is there life, but it has evolved quite a ways.â€
When that life actually began is anybody’s guess at this point, and Schopf is not confident that the record will ever be complete.
“There’s only one court of last resort in this business, and that is the rock record,†he said. “You are always constrained by your ability to find old rocks that have not been altered by heat.â€
Finding the oldest rocks has been a passion for many geologists, who determine age by the relative abundances of carbon isotopes, and they have searched most of the globe in that effort. Furthermore, if rocks older than 3.5 billion years are found, they are likely to have been severely altered by the forces of nature.
So will rocks bearing fossils from the first organisms ever be found?
“It doesn’t seem likely,†he said.
